Apparatus for carrying out a physical and/or chemical process, such as heat exchanger

ABSTRACT

An apparatus for carrying out a physical and/or chemical process, such as a heat exchanger, comprising a fluidized bed, through which a fluid flows, and a separator for separating fluidized bed particles from the medium for recirculation. According to the invention, the top box is provided with separated discharge means for discharging a main flow of medium with a partial flow of medium and for discharging granular material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 of PCT/NL00/00678, filed Sep. 22, 2000.

FIELD OF THE INVENTION

This invention relates to an apparatus for carrying out a physicaland/or chemical process, such as a heat exchanger, comprising areservoir provided with upwardly directed tubes, which tubes areaccommodated, at top and bottom ends thereof, in tube plates and are inopen communication with a top box and a bottom box, in which bottom boxat least one distribution plate is arranged for supporting a fluidizedbed of granular material, which is maintainable in a quasi-stationary,fluidized condition by a medium to be treated or heated, fed via thebottom box and flowing through the tubes, which apparatus furthercomprises a downcomer, placed outside the reservoir, for feedinggranular material separated from the medium back to the bottom box.

BACKGROUND OF THE INVENTION

Such apparatus is known from GB 2 087 538 and comprises a lock system inthe external downcomer forfeeding particles batchwise from the top boxback to the bottom box.

EP 0 065 332 and EP 0 451 518 disclose a fluidized bed type heatexchanger in which the fluidized bed is maintained in quasi-stationary,fluidized condition by recirculation of both granular material andmedium through an internal downcomer from the top box back to the bottombox.

A further apparatus is known from the international patent applicationPCT/NL94/00081. In this apparatus, the external downcomer placed outsidethe reservoir is connected at a top end with the top box via aseparator, designed as a cyclone, for separating granular material fromthe medium. At a bottom end, the downcomer is communicable, throughswitching on and off, with the bottom box of the reservoir.

A variant of the above apparatus, which is particularly suitable forprocessing a medium of high viscosity and/or a large volume flow ofmedium, is described in PCT/NL94/00082. In this apparatus, the bottombox is provided with a divided feed of medium, so that a part of themedium is fed above the fluidized bed, as a result of which thefluidized bed can be stable and sufficiently dense. In this variant, thetubes are provided with run-in pieces, extending into the bottom box,with inflow openings arranged in the tube walls for increasing theinflow of medium.

In the apparatuses known from PCT/NL94/00081 and PCT/NL94/00082, thetotal amount of flowing medium with fluidized bed particles includedtherein therefore flows from the vertical tubes of the reservoir via thetop box to the separator designed as a cyclone, in which the granularmaterial, hereinafter also called fluidized bed particles, is separatedfrom the medium.

A drawback of this arrangement is that the cyclone, in particular withlarge throughputs of flowing medium, can assume a considerable dimensionand that the walls of the separator and the communication line betweenthe separator and the top box must be protected against wear through thefluidized bed particles present in the flowing medium. The making ofarrangements against wear for such a relatively large cyclone separatorcan be very expensive, in particular when the flowing medium is alsocorrosive.

Furthermore, since the separator must be located at about the sameheight as the top box, the large cyclone separator must be supportedseparately. Since the reservoir with the top box often reaches aconsiderable height, the support is often relatively complicated andexpensive.

Another drawback of a cyclone as separator is the pressure drop requiredfor effecting separation of the fluidized bed particles and flowingmedium in the cyclone. When the material of these particles has a lowdensity, e.g. 2400 kg/cm³, and the dimensions of these particles arerelatively small, e.g. 1 to 2 mm, this pressure drop can impede thedownward transport of the fluidized bed particles in the externaldowncomer and even cause it to reverse to an undesirable upwardtransport, as a result of which the fluidized bed particles are nolonger fed to the bottom box, but discharged with the flowing medium viathe separator.

BRIEF SUMMARY OF THE INVENTION

The object of the invention is to provide an apparatus of the typementioned in the opening paragraph, in which the above drawbacks areremoved. According to the invention, the apparatus is thereforecharacterized in that the downcomer is provided near its top end with adischarge line for feeding the partial flow of medium from which thegranular material has been separated under the action of gravity, to themain flow of medium.

In the top box, the upward flow velocity of the medium is lower than inthe tubes. As a result thereof, depending on the selected upward flowvelocity and the viscosity of the medium, the form and weight of thegranular material, the granular material entrained via the tubes withthe medium from the fluidized bed can be separated from the medium in alower part of the top box under the action of gravity, while the mediumfrom which the granular material has been separated flows upward furtherto an upper part of the top box. The upward flow velocity of the mediumis therefore selected to be lower than the downward fall velocity of thegranular material in the medium.

By means of the first discharge, the granular material separated in thetop box can be discharged with a partial flow of the medium to thedowncomer, while with a second discharge located downstream above a mainflow of medium can be discharged free from particles.

The partial flow of medium ensures that the discharge of the granularmaterial through the first discharge line can proceed reliably and withsufficient throughput. To stimulate flow of the partial flow of mediumwith the granular material through the first discharge line, theapparatus can be provided with means for generating a pressuredifference over the first discharge line, such that the pressure nearthe entrance of the first discharge line is higher than near the exit ofthe first discharge line.

Such means for generating a pressure difference may comprise a flowresistance provided in the top box, e.g. a distribution plate in the topbox, a fluidized bed extending with a height h above the entranceopening of the first discharge, an entrance loss and/or a lineresistance in the second discharge line.

Furthermore, at the location of the outflow opening of the firstdischarge line, the downcomer may be connected with an area with apressure lower with respect to the entrance opening of the firstdischarge line, e.g. a part of the second discharge line locateddownstream.

In an elegant manner, the top box may be divided by means of adistribution plate into a lower part to which the first discharge lineis connected and a second part where, as a result of the distributionplate, a lower pressure prevails, and to which the second discharge lineis connected, while an upper part of the downcomer is connected with theupper part of the top box.

From the top box, the separated granular material is fed to thedowncomer where it falls down under the action of gravity.

The granular material is fed to the external downcomer with a small andadjustable partial flow of flowing medium. Within the downcomer, aseparation of flowing medium and granular material is then effectedagain under the action of gravity. The partial flow of flowing medium,from which the granular material has been separated, can be dischargedby generating only a small pressure difference via the top end of theexternal downcomer. In an advantageous embodiment, the externaldowncomer can therefore be provided near its top end with a dischargeline which, viewed in the direction of flow of the medium in the topbox, is connected with the top box between the first and the seconddischarge line. Of course, a line for feeding the partial flow of mediumfrom which the granular material has been separated to the main flow canalso be provided in another manner.

The separator, designed as a cyclone, arranged externally with respectto the top box, can be avoided according to the invention, with theresult that the use of this relatively large additional apparatus andthe additional provisions required therefore against wear is no longernecessary. Moreover, no separate supporting structure is required.Because the flow velocity reducing means are integrated into the topbox, the supporting structure already present for the top box and thereservoir suffices. Furthermore, only a small pressure drop is necessarybetween the top box and the external downcomer, so that the risk ofundesirable upward transport of granular material in the downcomer issubstantially lower.

By using the means, integrated with the top box, for reducing the flowvelocity, it is ensured that even at greater throughputs and/or highviscosities, during use, the upward flow velocity of the medium in thetop box can be lower than the fall velocity of the granular material inthe flowing medium, as a result of which the granular material can beseparated.

The flow velocity reducing means preferably comprise an expansion spaceintegrated with the top box, e.g. a part of the top box having anenlarged diameter with respect to the reservoir. This ensures that thedimensions of the top box with expansion space in upward direction canbe relatively small.

In addition to or side by side with that, the flow velocity reducingmeans may comprise a flow resistance integrated with the top box, e.g. adistribution plate extending between the first and the second dischargeopening in the top box and provided with flow passage openings. The flowresistance may already be formed by a second fluidized bed of granularmaterial. Such a bed can be formed, during use, in e.g. the top boxabove a supporting plate provided in the top box, e.g. designed as athrottling plate placed above the openings of the tubes. Preferably, thefirst discharge opening, during use, connects to the fluidized bed,while the second discharge opening is located in upward direction at adistance above the fluidized bed.

The invention also relates to a method for carrying out a chemicaland/or physical process.

The manner of feeding flowing medium to the bottom box of the apparatusand feeding particles of granular material back to the bottom box viathe downcomer is already known to those skilled in the art and can becarried out in many ways, e.g. as described in the international patentapplications PCT/NL94/00081 and PCT/NL94/00082.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention will now be explained in more detail with reference to anexemplary embodiment shown in a drawing. In the drawing:

FIG. 1 is a diagrammatic cross-section of a heat exchanger according tothe invention; and

FIG. 2 is a detail of a top end of the downcomer of the heat exchangerof FIG. 1, in which a separation wall is provided.

FIG. 3 is a detail of the top end of a second embodiment of the heatexchanger of FIG. 1, in which a tube is provided with openings.

The figures only relate to diagrammatic representations of preferredembodiments of the invention. In the figures, similar or correspondingparts are indicated by the same reference numerals.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a heat exchanger comprising a reservoir 1, in which aseries of risers 2 are provided, which are accommodated at the top end2A and the bottom end 2B in tube plates 2C, 2D. Above the upper tubeplate 2C a top box 4 separated from the reservoir 1 is located, intowhich the risers 2 open, while below the lower tube plate 2D a bottombox 3 separated from the reservoir is present, in which one or moredistribution plates 5 are provided to support a fluidized bed ofgranular material. The risers 2 are provided at the bottom ends 2B withrun-in pieces 6, in which openings 7 are provided. Arranged above or ata short distance from the upper tube plate 2C is a throttling plate 9with openings 8, which are in register with the openings of the risers 2and have a diameter smaller than the internal diameter of the risers 2.In the top box 4 are provided an opening 12, to which connects a firstdischarge line 13 for discharging, with a partial flow of medium fromthe top box 4, fluidized bed particles separated in the top box, and anopening 10. In the top box 4 is further provided a second discharge line11, located upstream in the upward direction of flow of the mediumindicated by the arrow P, for discharging a main flow of medium withoutgranular material. The first discharge opening 12 is connected with thetop end of the external downcomer 14. The medium to be treated or heatedis fed to the heat exchanger via the line 20, which divides into a line21A to the bottom box 3 and a line 21B to the collecting reservoir 22located at the bottom end of the external downcomer 23. By means of adistribution plate 24 for supporting the fluidized bed particles fedthrough the external downcomer, the collecting reservoir 22 is dividedinto a bottom section 25 and a top section 26. The top section 26 isconnected via one or more lines 27 to the bottom box 3 for feeding thefluidized bed particles back thereto from the collecting reservoir 22.The lines 20, 21B and 27 may be provided with regulating or shut-offvalves.

In the outlet box 4 a division of the discharge of flowing medium takesplace. The largest amount of flowing medium, the main flow, flowsthrough the outlet box 4 substantially in an upward direction, and isdischarged via opening 10 and the second discharge line 11. A muchsmaller amount of flowing medium, the partial flow, with the totalamount of circulating fluidized bed particles therein, is dischargedfrom the outlet box 4 substantially sideward via opening 12 and thefirst discharge line 13 to the top end of the external downcomer 14. Toprevent a relatively large main flow of flowing medium from entrainingfluidized bed particles from the outlet box 4, the outlet box is, inthis example, provided with flow velocity reducing means for reducingthe upward flow velocity of the medium, designed as an integratedexpansion space. At the location of the expansion space the top box 4has a greater diameter than the reservoir 1. The top box 4 with theexpansion space is dimensioned such that, during use, the velocities ofthe flowing medium remain so low that discharge of fluidized bedparticles via the second discharge line is not possible.

The size of partial flow of flowing medium may be adjusted by providinga flow resistance in the expansion space between the first dischargeline 13 and the second discharge line 11, e.g. by including in theoutlet box 4 a distribution plate 15 provided with flow passageopenings. The main flow must pass this distribution plate, which has theresult that this pressure difference arises. The partial flow does notpass this plate and can be short-circuited via line 13, the upper partof the downcomer 14 and line 16 with the space behind this plate where alower pressure prevails. Depending on the dimensioning of this plate, agreater or smaller pressure difference arises. A great pressuredifference gives a large partial flow. This can be reduced again byproviding a regulating valve 17 in line 16.

This small partial flow transports all circulating fluidized bedparticles from the outlet box 4 to the upper part of the downcomer 14.To prevent fluidized bed particles from being entrained with the partialflow directed upwards in the downcomer, indicated by arrow Q, this upperpart of the downcomer is provided with such a diameter that the upwardlydirected velocity of flowing medium in this part of the downcomer issubstantially lower than the fall velocity of one single fluidized bedparticle in the flowing medium.

Of course, other means may also be included in the top box 4 to generatea flow resistance, e.g. the height h of the level of the fluidized bedin the outlet box above opening 12, a line resistance, a hole plate, agrid, a screen plate, an orifice and/or an entrance loss through a line.A proper choice of the dimension of the fluidized bed particles, thedensity of the material of the fluidized bed particles, the partial flowof flowing medium and the cross-section of the upper part of thedowncomer 14 renders it possible that in the upper part of the downcomer14 particles other than fluidized bed particles, the so-called‘fluidized bed-foreign’ particles, are separated from the fluidized bedparticles and discharged with the upwardly directed partial flow. This‘fluidized bed-foreign’ particles are e.g. particles already present inthe flow of flowing medium fed to the heat exchanger or formed in theheat exchanger, such as e.g. cyrstals. Thus, crystals with the fluidizedbed particles flowing downward in the downcomer can be prevented frombeing fed again to the inlet box.

Sometimes, deposits may be formed on the walls of the outlet box 4,which are not subject to the scouring and cleaning effect of thefluidized bed particles present in the outlet box. These deposits cangradually grow and decompose, and the decomposed pieces can then assumesuch dimensions that they cannot be discharged from the outlet box 4with the main flow of flowing medium. As a result of the fluidization ofthe fluidized bed particles in the outlet box 4 and the attendantgrinding effect, the decomposed deposits, in fact also ‘fluidizedbed-foreign’ particles, will be slightly reduced, but, simultaneously,these particles move with the particles of the circulating fluidized bedto the opening 12 of line 13, which connects the outlet box 4 with theupper part of the downcomer 14. To prevent too large parts of thedecomposed deposits from entering the downcomer 14, the opening 12 maybe provided with a screen, e.g. a screen plate 12A, in which openingsare provided, which can pass the fluidized bed particles and thosepieces of deposit material meanwhile sufficiently reduced by thegrinding effect of the fluidized bed particles. If this is not the case,these pieces of deposit material are kept longer in the fluidized bed inthe outlet box, as a result of which these pieces can be reduced furtheruntil these parts, too, can pass through the openings of the screenplated or cage-shaped screen 12A. If the ground pieces of depositmaterial fed via line 13 to the upper part of the downcomer 14 are stilltoo large to be discharged with the partial flow of flowing medium,these parts will be entrained with the fluidized bed particles flowingdownward in the downcomer, fed to the inlet box and then, via the tubes,the outlet box and line 13, enter the upper part of the downcomer again.During this transport of the particles from deposit material, incontinuous contact with the fluidized bed particles, a further reductionwill take place by grinding or pulverization. Finally, the particles ofdeposit material will have been reduced so far that they can bedischarge from the upper part of the downcomer with the partial flow offlowing medium.

Moreover, in the inlet box 3 above the connection of line 27, throughwhich the fluidized bed particles and particles of deposit material canbe fed to the inlet box, a screen plate 5A may be provided. This extendsthe residence time of the particles consisting of deposit material inthe grinding and pulverizing fluidized bed in the inlet box, whichresults in a further reduction of these particles. The above-describedmethod for reducing the pieces of deposit material formed in the outletbox for the purpose of finally being able to discharge this materialwith the partial flow of flowing medium is also applicable to depositmaterial entering the heat exchanger but formed elsewhere, e.g. in theinlet box or in the lines upstream of the heat exchanger. It may beclear that such screens 12A, 5A may be designed in many ways and may beused in top boxes or bottom boxes of other types of heat exchangers,e.g. as generally described in PCT/NL94/00081.

Furthermore, in line 16 may be included a separator 18, with which‘fluidized bed-foreign’ particles above a specific dimension can beremoved from the partial flow via line 19. Discharge of these particlesmay take place continuously or batchwise. Such a separator can berealized with relatively limited dimensions, because the partial flow issmall with respect to the main flow. In general, the partial flow willbe discharged to that part of the outlet box 4 where a lower pressureprevails. That is, in the present case, downstream of the distributionplate 15, or in case that no distribution plate 15 is used to line 11where, too, a pressure lower than in the outlet box prevails as a resultof the height h of the level of the fluidized bed in the outlet boxabove the opening 12, the entrance loss and line resistance. Theadvantage of the above-described situation is that the partial flow iscoupled to the main flow. When the main flow increases, the pressuredifference also increases, and hence the partial flow, too. Of course,pressure difference and partial flow will decrease when the main flowdecreases.

In some cases, the partial flow of flowing medium is not to be fed backto the main flow of flowing medium, but to be discharged to another partof the process with a pressure much lower than in the outlet box. Thisrequires a good regulation for the partial flow, since otherwise thereis a risk that too large a partial flow will be withdrawn from the heatexchanger, which involves a risk that with the partial flow fluidizedbed particles will be discharged from the heat exchanger, too. It hasbeen observed before that the pressure difference required for realizingthe partial flow from the outlet box 4 via line 13, the upper part ofthe downcomer 14 and line 16 may be about 0.05 to 0.1 bar. By properlydesigning the upper part of the downcomer 14, this pressure differencecan be reduced. Optionally, the desired partial flow may be realizedwithout making use of the distribution plate 15 or of the entrancepressure loss in opening 10, whether or not in combination with thepressure loss in a part of line 11.

FIG. 2 therefore shows an embodiment of the upper part of the downcomer14, by which the above pressure loss can be limited or avoided.

For this purpose, the upper part of the downcomer 14 is divided into twochannels. A first channel 14A, connected to line 13 and closed at thetop end and open at the bottom end, processes the downward partial flowof flowing medium coming from the outlet box 4 with the circulatingfluidized bed particles and ‘fluidized bed-foreign’ particles presenttherein. A second channel 14B, the cross-section of which is determinedby the cross-section of the upper part of the downcomer 14, reduced bythe cross-section of channel 14A, processes the upward partial flow offlowing medium with ‘fluidized bed-foreign’ particles optionally presenttherein. The upwardly directed velocity of the partial flow of flowingmedium in channel 14B must be lower than the fall velocity of one singlefluidized bed particle in the flowing medium. By designing channel 14Awith a smaller cross-section than channel 14B, a high concentration offluidized bed particles is formed in channel 14A. This contributes to anatural circulation of the flowing medium from the outlet box 4, vialine 13, channel 14A, channel 14B and finally via line 16 to the pointof connection of line 16 to the outlet box 4 or line 11. The greater thelength of channel 14A is selected, the more the driving force for thenatural circulation of the partial flow of flowing medium increases. Ata sufficient length of channel 14A and a sufficiently high concentrationof fluidized bed particles in channel 14A, it can be decided not to usea pressure difference over a distribution plate or a pressure differenceowing to an entrance loss, or possibly in combination with a pressuredifference as a result of a line resistance. It is observed that such adowncomer provided with two channels can, also in itself, beadvantageously used in other heat exchangers, e.g. of the type describedin PCT/NL94/00081.

As shown in the exemplary embodiment, the channels 14A, 14B can beformed by a separation plate 28 extending downwardly in the upper partof the downcomer 14. The channels may also be formed in other ways, forinstance by providing in the upper part of the downcomer a downwardlyextending tube, with which the fluidized bed particles are fed to thedowncomer. Preferably, such a tube is formed as extension of the seconddischarge line 13.

If the partial flow of flowing medium is to be operated completely onnatural circulation, the line 13 may be arranged at a downward angle toset the natural circulation in self-starting motion.

The upper part of the downcomer 14 should preferably be able not only toaccommodate the cross-sections of the channels 14A and 14B, but also toserve as reservoir for the storage of fluidized bed particles. Duringnormal use, the downcomer 14 is preferably filled about half with apacked bed of fluidized bed particles. In this example, during normaluse, this packed bed of circulating fluidized bed particles preferablyhas a rate of about 10 to 15 cm/s in the lower part of the downcomer 23.The amount of circulating fluidized bed particles and the rate of thebed of particles in the downcomer determine the cross-section of thelower part of the downcomer 23. If the flow of flowing medium throughline 21B is closed, the transport of fluidized bed particles from thedowncomer to the inlet box will stop, too. The flow of flowing mediumthrough line 21A will, however, flush a great part of the fluidized bedparticles out of the inlet box 3, tubes 2 and outlet box 4 via line 13and channel 14A to the upper part of the downcomer. The space belowchannels 14A is generally not sufficient to accommodate this amount offluidized bed particles, since the aim is to maximize the length ofchannel 14A. It may therefore happen that channel 14A, too, is filled upwith fluidized bed particles coming from the inlet box, the tubes andthe outlet box of the heat exchanger. Preferably, channel 14A isprevented from being filled up until the outlet of line 13 one or moreopenings 29 may be provided in the separation wall between channel 14Aand channel 14B, thus ensuring that fluidized bed particles can overflowfrom channel 14A into channel 14B and the full cross-section of theupper part of the downcomer can be utilized for storage of fluidized bedparticles. As shown in FIG. 3, a tube may extend downward in thedowncomer including overflow openings 29 in another embodiment. In themajority of cases, the demands to accommodate channel 14A and channel14B in one cross-section of the upper part of the downcomer 14 incombination with the demands for storage a specific amount of fluidizedbed particles will lead to a cross-section greater than thecross-section of the lower part of the downcomer 23, which is determinedby the velocity of the packed bed. This accounts for the steppedconfiguration of the cross-section of the downcomer 14.

It may be clear that the invention is not limited to the preferredembodiment shown herein.

1. An apparatus for carrying out a physical chemical process on amedium, the apparatus comprising: a reservoir, a tube disposed in thereservoir and having a first end portion and a second end portion, tubeplates, wherein the first end portion and second end portion of the tubeare received in the tube plates, a first box and a second box, whereinthe first end portion of the tube is in open communication with thefirst box and the second end portion of the tube is in opencommunication with the second box, a first distribution plate disposedin the second box, wherein the first distribution plate supports a firstfluidized bed of granular material, wherein said first fluidized bed ofgranular material is maintainable in a quasi-stationary, fluidizedcondition by the medium, wherein the medium flows through the second boxand the tube and entrained granular material is separated from themedium in the first box by gravity, a downcomer disposed outside thereservoir, a first discharge line provided in the first box, dischargingthe separated granular material and a partial flow of the medium to thedowncomer, wherein the separated granular material is further separatedfrom the partial flow of the medium in the downcomer by gravity, asecond discharge line provided in the first box, discharging a main flowof the medium, wherein the granular material has been separated from themain flow of the medium, and a third discharge line provided near afirst end in the downcomer, feeding the granular free partial flow ofthe medium to the main flow of the medium, and wherein said downcomerfeeds the granular material separated from the partial flow medium tothe second box; wherein the downcomer is provided near the first endwith a first channel for downward transport of the granular material fedto the downcomer and a second channel for upward transport of thepartial flow of the-medium without granular material, and wherein thedowncomer is provided with a downwardly extending separation plate,forming the first channel and the second channel of the downcomer. 2.The apparatus of claim 1, wherein the first box comprises integratedflow velocity reducing means, reducing an upward flow velocity of themedium.
 3. The apparatus of claim 2, wherein the flow velocity reducingmeans comprise an expansion space.
 4. The apparatus of claim 3, whereina part of the first box defines the expansion space and has an enlargeddiameter with respect to the reservoir.
 5. The apparatus of claim 2,wherein the flow velocity means comprise a flow resistance.
 6. Theapparatus of claim 5, wherein the flow resistance is at least partlyformed by a second fluidized bed of granular material, wherein thesecond fluidized bed of granular material is formed in the first box. 7.The apparatus of claim 6, wherein the first discharge line connects tothe second fluidized bed, and wherein the second discharge line islocated in an upward flow direction at a distance above the secondfluidized bed.
 8. The apparatus of claim 5, wherein the flow resistancecomprises a second distribution plate, extending in the expansion spacebetween the first and the second discharge lines, and defining flowpassage openings.
 9. The apparatus of claim 1, wherein an opening of thefirst discharge line is provided with a screen.
 10. The apparatus ofclaim 1, wherein the second box is provided with a screen.
 11. Theapparatus of claim 1, wherein the apparatus is a heat exchanger.